Photosensitive slurry

ABSTRACT

The present invention aims to solve problems involved in the formation of a conductive or insulating layer in a pattern form by photolithography, i.e., an environmental problem associated with handling a solvent and a problem associated with wastewater treatment in the development with an aqueous alkaline solution. A method for forming a conductive layer (an anode bus 3) or an insulating layer (a barrier 1) on a glass substrate by photolithography using a photosensitive slurry solution prepared by mixing a low-melting glass powder as a binder and a conductive or insulating powder into a PVA-based, water-soluble photosensitive solution, wherein the content of B 2  O 3  component in the whole low-melting glass powder is closely regulated to not more than 6% by weight. This enables coating without gelation of PVA. Unlike the conventional photolithography using a solvent type photosensitive slurry, the method of the present invention can solve the environmental problem associated with handling a solvent and the problem associated with wastewater treatment in the development with an aqueous alkaline solution.

This is a division of application Ser. No. 08/480,608 filed Jun. 8,1995, and now U.S. Pat. No. 5,672,460.

BACKGROUND OF THE INVENTION

The present invention relates to a technique which can be applied mainlyto a self-luminescent gas discharge display panel comprising a glasssubstrate bearing a conductive layer and an insulating layer and,further, to a thermal head comprising glass having a conductive layerand a heater used in a rear window of automobiles. More particularly,the present invention relates to a method for forming a conductive orinsulating layer in a pattern form on a glass substrate byphotolithography.

The present invention will now be described by taking a DC-type gasdischarge display panel as an example.

FIG. 1 is an embodiment of the construction of a DC-type plasma displaypanel (hereinafter referred to as "PDP"). In the DC-type PDP, a flatfront plate (not shown) and a back plate (not shown) each formed of aninsulating material, such as glass, are provided parallel to and so asto face each other. A barrier 1 is fixed on the inner side of the backplate in a direction orthogonal to the back plate. This barrier 1 servesto provide a certain gap between the front plate and the back plate. Acathode 2 is formed on the inner side of the front plate. On the otherhand, an anode bus 3 and a resistor 4 are provided on the inner side ofthe back plate in a direction orthogonal to the cathode 2. Further, apad 5 is provided on the resistor 4, and a fluorescent screen 6 isprovided adjacent to the side face of the pad 5. The anode bus 3 isgenerally formed of gold or silver, the resistor 4 is generally formedof ruthenium, and the cathode 2 is generally formed of nickel or thelike.

In the DC-type PDP having the above construction, the application of apredetermined voltage from a direct current power supply across the pad5 and the cathode 2 creates an electric field, causing discharge withineach discharge cell 7 defined by the front plate, the back plate, andthe barrier 1. The discharge produces ultraviolet light which rendersthe fluorescent screen 6 luminous, and a viewer perceives the lightpassing through the front plate.

Screen printing is known as a method for forming an anode bus 3 and abarrier 1 on a back plate of PDP having a structure shown in FIG. 1.Another method known in the art is photolithography wherein alow-melting glass powder and a conductive or insulating powder are addedto a photosensitive solution using an organic solvent as a solventsystem to prepare a photosensitive slurry which is then coated on aglass substrate, dried, exposed, and developed with an aqueous alkalinesolution to form a desired pattern. Further, as a method for forming theanode bus 3, a technique is known wherein a conductive paste isscreen-printed to provide a solid print which is fired and then etched.

In an attempt to increase the size and fineness in PDP, the possiblefinest anode bus and barrier attained by the conventional screenprinting are about 100 μm in width. Further, the conventional method hasa problem that elongation and strain in a screen plate render thepattern inaccurate. This problem brings about a local difference inresistance between the anode bus and the pad, resulting in unevendischarge characteristics. On the other hand, the technique where ananode bus is formed by etching suffers from a complicated productionprocess and, in addition, poses a problem of wastewater treatmentinvolved in the etching.

In view of the above problems, photolithography is considered suitablefor the formation of a conductive or insulating layer. The aboveconventional photolithography involves an environmental problem, whichunavoidably occurs in handling a solvent, and a problem of wastewatertreatment associated with the development with an aqueous alkalinesolution.

DISCLOSURE OF THE INVENTION

The present invention aims to eliminate the above problems of the priorart, and an object of the present invention is to provide a method forforming a conductive or insulating layer, which can solve all theproblems involved in the formation of a conductive or insulating layerin a pattern form on a glass substrate by the conventionalphotolithography, that is, an environmental problem, which unavoidablyoccurs in handling a solvent, and a problem of wastewater treatmentassociated with the development with an aqueous alkaline solution.

In order to attain the above object, the method for forming a conductiveor insulating layer according to the present invention comprises thesteps of: coating a substrate with a photosensitive slurry comprising amixture of a polyvinyl alcohol(PVA)-based, water-soluble photosensitivesolution with a low-melting glass powder as a binder and a conductive orinsulating powder; drying the resultant coating; subjecting the driedcoating to exposure and development with water; and firing the developedcoating to form a conductive or insulating layer in a desired patternform on the substrate, said low-melting glass powder having a content ofB₂ O₃ as a component thereof of not more than 6% by weight based on thewhole low-melting glass powder.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a typical view of an embodiment of the construction of aDC-type plasma display panel as a gas discharge display panel.

BEST MODE FOR CARRYING OUT THE INVENTION

The method for forming the conductive layer/insulating layer accordingto the present invention comprises the steps of: coating a substratewith a photosensitive slurry comprising a mixture of a polyvinylalcohol(PVA)-based, water-soluble photosensitive solution with alow-melting glass powder as a binder and a conductive or insulatingpowder; drying the resultant coating; subjecting the dried coating toexposure and development with water; and firing the developed coating toform a conductive or insulating layer in a desired pattern form on thesubstrate, said low-melting glass powder having a content of B₂ O₃ as acomponent thereof of not more than 6% by weight based on the wholelow-melting glass powder.

The present inventors have aimed at the use of a photosensitive slurrycomprising a mixture of a PVA-based water-soluble photosensitivesolution with a low-melting glass powder as a binder and a conductive orinsulating powder with a view to developing a photolithographic processwhich can eliminate the need to use a solvent-type photosensitivesolution and render the wastewater from the development step harmless,and have made extensive and intensive studies. As a result, they havefound the following facts.

Specifically, in the preparation of a photosensitive slurry by adding alow-melting glass powder and a conductive or insulating powder to aphotosensitive solution composed mainly of PVA, the use of a powderhaving a content of B₂ O₃ as a component of 10% by weight (for example,OC-535 manufactured by Okuno Chemical Co., Ltd.) as the low-meltingglass powder results in gelation of PVA, making it impossible to use thephotosensitive slurry. By contrast, it was found that a glass powderhaving a B₂ O₃ content of 4% by weight (for example, OC-200 and Z-7000manufactured by Okuno Chemical Co., Ltd.) can provide a goodphotosensitive slurry without gelation of PVA.

The present inventors have made further studies based on this findingand, as a result, have found that the gelation is significantly reducedin a critical manner with a B₂ O₃ content of the low-melting glasspowder of 6% by weight as the border. Although the reason why thegelation occurs has not been fully elucidated yet, it is believed toreside in that the B₂ O₃ component, which has been eluted from thelow-melting glass powder into the photosensitive solution, or the B₂ O₃component present on the surface of the powder particles is reacted withPVA, causing gelation. For example, the formation of a water-insolubleresin coating on the surface of the glass particles for the purpose ofinhibiting the elution of the B₂ O₃ component is effective forpreventing such gelation of the aqueous PVA solution. The formation ofsuch coating on the surface of the glass particles complicates theproduction process and, in addition, is disadvantageous also from theviewpoint of cost.

The present invention has been made based on the above results ofstudies and is characterized in that the above problems have beeneliminated by closely regulating the content of the B₂ O₃ -component inthe low-melting glass powder to not more than 6% by weight based on thewhole low-melting glass powder.

In the method for forming a conductive or insulating layer according tothe present invention having the above constitution, a photosensitiveslurry which causes no gelation reaction derived from a low-meltingglass powder as a binder mixed into the aqueous PVA-based solution canbe provided, and the formation of a pattern by photolithography iscarried out using this photosensitive slurry of non-organic solventtype. Further, since water is used as a solvent in the step ofdevelopment, unlike the conventional method, no problem of wastewatertreatment occurs.

As described above, in the present invention, in principle, water isused as the solvent, and no organic solvent is used.

In the present invention, a photosensitive solution having the followingcomposition is used as the PVA-based, water-soluble photosensitivesolution.

    ______________________________________    PVA                0.5 to 20% by weight    Water              76 to 99.47% by weight    Photosensitizing agent                       0.03 to 4% by weight    ______________________________________

Specific preferred examples of the sensitizing agent include diazocompounds and ammonium bichromate.

The low-melting glass powder composition as a binder to be mixed intothe aqueous PVA-based solution may be the same composition as commonlyused as a low-melting glass powder except that the content of the B₂ O₃is strictly limited to the above content range. Specific preferredexamples thereof include SiO₂, Al₂ O₃, and PbO.

The particle diameter of the glass powder is preferably in the range offrom about 2 to 7 μm.

In the present invention, the content of the B₂ O₃ component in thelow-melting glass powder is limited to not more than 6% by weight,preferably not more than 5% by weight, still preferably not more than 4%by weight.

When the content of the B₂ O₃ component exceeds 6% by weight, theresultant photosensitive slurry gels, making it difficult to form auniform coating Further, in the present invention, B₂ O₃ is a componentwhich is substantially effective for preparing low-melting glass andgenerally present in an amount of preferably not less than 1% by weight.

The amount of the low-melting glass powder incorporated is preferably0.2 to 30% by weight based on the total amount of the photosensitiveslurry.

Preferred conductive materials for forming the conductive layer includeAu, Ag, RuO₂, ITO, Al, Ti, Cr, Fe, Ni, Cu, Zr, Nb, Mo, Pb, Ta, W, andPt. When the conductive material is used as an electrode, Au, Ag, RuO₂,and ITO (Indium-Tin Oxide ) are particularly preferred.

Preferred insulating materials for forming the insulating layer includealumina, high-melting glass powders, and various pigments.

The kind and the amount of these materials may be properly selectedaccording to the applications.

The photosensitive slurry thus prepared is coated on a substrate, suchas glass, and the resultant coating is then dried, subjected topattern-wise exposure, developed with water, and fired to prepare asubstrate bearing a conductive or insulating layer in a desired patternform.

The substrate having a conductive pattern or the substrate having aninsulating pattern thus prepared is applicable to a discharge displaypanel, a thermal head, a rear window and the like according tocontemplated applications.

EXAMPLE 1

The present example demonstrates the formation of a conductive layer.Specifically, the formation of an anode bus comprising a gold electrodein a DC-type PDP shown in FIG. 1 will now be described.

At the outset, a solution having the following composition was preparedas a photosensitive slurry.

    ______________________________________                     (wt %)    ______________________________________    Low-melting glass powder                       0.60    Gold powder        11.40    PVA                0.95    Photosensitizing agent                       0.15    Deionized water    86.90    ______________________________________

The low-melting glass powder was Z-7000 (B₂ O₃ content: 4%) manufacturedby Okuno Chemical Co., Ltd., PVA was PVA224 manufactured by Kuraray Co.,Ltd., and the photosensitizing agent was a diazo compound. In theresultant slurry, no gelation of PVA was observed, and the low-meltingglass powder and the gold powder were in an evenly dispersed state.

Then, in order to make the above photosensitive slurry coatable on aglass substrate by means of a blade coater, a thickening agent (forexample, Metlose (90SH-4000) manufactured by The Shin-Etsu Chemical Co.,Ltd.) is added to the photosensitive slurry to adjust the viscosity. Thephotosensitive slurry is then coated on a glass substrate by means of ablade coater, and the resultant coating was dried to form a 10 μm-thickdried coating. Coating may be carried out by any method so far as aneven coating can be formed on a glass substrate. Subsequently, thesample thus obtained was exposed to ultraviolet light through anexposure mask and developed with water to provide a desired electrodepattern. It was then fired at about 580° C. to bake the pattern on theglass substrate, thereby forming an anode bus comprising a goldelectrode.

EXAMPLE 2

The present example demonstrates the formation of an insulating layer.Specifically, the formation of a barrier in a DC-type PDP shown in FIG.1 will now be described.

At the outset, a solution having the following composition was preparedas the photosensitive slurry.

    ______________________________________                     (wt %)    ______________________________________    Low-melting glass powder                       10.20    Alumina powder     1.80    PVA                0.95    Photosensitizing agent                       0.15    Deionized water    86.90    ______________________________________

All the materials except for the alumina powder were the same as thoseused in Example 1. The alumina powder particles used were spherical andhad a diameter of about 5 μm. In the resultant slurry, no gelation ofPVA was observed, and the low-melting glass powder and the aluminapowder were in an evenly dispersed state.

The viscosity of the above photosensitive slurry was regulated in thesame manner as in Example 1 and coated by means of a blade coater, andthe resultant coating was dried and exposed to ultraviolet light throughan exposure mask. The above series of steps of coating, drying, andexposure were repeated to provide a desired coating thickness, anddevelopment with water was carried out at one time. Firing was thencarried out to form a barrier having a height of 200 μm.

Although the formation of an anode bus (a conductive layer) and abarrier (an insulating layer) has been described above by taking DC-typePDP as an example, the present invention is applicable also to all theother types of PDP and, further, to a thermal head comprising glasshaving a conductive layer and a heater used in a rear window ofautomobiles.

EXAMPLE 3

A low-melting glass powder (sample T101-6), having a B₂ O₃ content of6.0% by weight, prepared on an experimental basis was added to and mixedwith a photosensitive solution composed mainly of PVA in the same manneras in Examples 1 and 2. In the resultant photosensitive slurry, nogelation of PVA was observed, and the low-melting glass powder and thegold powder or the alumina powder were in an evenly dispersed state.

Comparative Example 1

In order to prepare a photosensitive slurry, the procedure of Examples 1and 2 was repeated, except that a low-melting powder (OC-535, B₂ O₃content 10%, manufactured by Okuno Chemical Co., Ltd.) was used. As aresult, PVA gelled, making it impossible to prepare a photosensitiveslurry.

Comparative Example 2

In order to prepare a photosensitive slurry, the procedure of theexamples was repeated, except that a low-melting powder (F-01, B₂ O₃content 13%, manufactured by Okuno Chemical Co., Ltd.) was used. As aresult, PVA gelled, making it impossible to prepare a photosensitiveslurry.

Comparative Example 3

A low-melting glass powder (sample T101-65), having a B₂ O₃ content of6.5% by weight, prepared on an experimental basis was added to and mixedwith a photosensitive solution composed mainly of PVA in the same manneras in Examples 1 and 2. As a result, PVA gelled, making it impossible toprepare a photosensitive slurry.

As described above, according to the present invention, a conductive orinsulating layer in a pattern form is formed by photolithography using aphotosensitive slurry of non-organic solvent type, and, further, waterrather than an aqueous alkaline solution is used in the step ofdevelopment. Therefore, unlike the conventional photolithography using aphotosensitive slurry of organic solvent type, the present invention cansolve an environmental problem associated with handling a solvent and aproblem of wastewater treatment associated with the development with anaqueous alkaline solution. Further, the method of the present inventionhas better suitability for a large-size/high-fine substrate than theconventional screen printing and, further, as compared with theconventional method wherein an electrode is formed by patterning usingetching, can simplify the production process and eliminate the problemof wastewater treatment involved in the etching.

We claim:
 1. A photosensitive slurry comprising a mixture of (i) apolyvinyl alcohol-based, water-soluble photosensitive solution, (ii) alow-melting glass powder as a binder, and (iii) one of a conductivepowder or an insulating powder, said low-melting glass powder having acontent of B₂ O₃ as a component thereof of not more than 6% by weightbased on the total amount of low-melting glass powder in said mixture.2. The photosensitive slurry according to claim 1, wherein thephotosensitive slurry is essentially free from an organic solvent. 3.The photosensitive slurry according to claim 1, wherein the conductivepowder comprises at least one member selected from the group consistingof Au, Ag, RuO₂, ITO, Al, Ti, Cr, Fe, Ni, Cu, Zr, Nb, Mo, Pb, Ta, W, andPt.
 4. The photosensitive slurry according to claim 1, wherein theinsulating powder comprises at least one of alumina, a high-meltingglass powder, and a pigment.
 5. The photosensitive slurry according toclaim 1, wherein the low-melting glass powder is a powder containingSiO₂, PbO, and Al₂ O₃.
 6. The photosensitive slurry according to claim1, wherein the polyvinyl alcohol-based, water-soluble photosensitivesolution contains as a sensitizing agent a diazo compound or ammoniumbichromate.
 7. The photosensitive slurry according to claim 1 whereinthe B₂ O₃ content is not more than 5% by weight based on the totalamount of low-melting glass powder in said mixture.